The empiric equations estimating the temperature dependences of average surface tension of high-angle grain boundaries in elemental solids are presented. The grain boundaries are considered as a homogeneous liquid-like layer in a solid matrix. The expressions present the grain-boundary surface tension as functions of the melting temperature, melting enthalpy, and molar volume of the elemental solid at the melting temperature. The empiric expressions relating grain-boundary surface tension with the surface tensions of elements in the solid and liquid states at melting temperature are obtained on this basis. The relations proposed can be useful to estimate the averaged surface tension of high-angle grain boundaries in elemental solids at high enough homologous temperature.